This invention relates to antenna systems with means for automatically correcting for depolarization effects such as Faraday rotation, satellite caused rotation and rain.
The reuse of frequency spectrum based on two orthogonal polarizations is vitally dependent on the achievable isolation between these polarizations. If the orthogonality of the two polarizations (perpendicularity in the case of linear polarizations and perfectly left and right-hand circular polarizations in the case of circular polarization) is ideal at the transmit end and no cross-polarized component is generated by the media of propagation, then the available isolation at the receive antenna depends on the cross-polarized level (axial ratio) of the receive antenna and the perfectness of alignment of the polarization with the incoming wave.
In a two-way communication system, usually the same antenna at different frequencies is used for receive and transmit communications. Since the polarization attitude for these two frequencies are generally different, perfect polarization matching requires separate polarization alignment for the two frequencies. This, for instance, can be done by simultaneously minimizing the cross-polarized power levels at each receiving end of the above-described communications link.
In satellite communications, however, such a technique is not convenient, since polarization alignment is required not only at the earth station but also at the satellite. The complexity of the spaceborne equipment for such purpose can be avoided if the receive and the transmit polarizations are adjustable simultaneously at the earth station. When the propagation media is not affecting the polarization attitude of the up and down link waves, such alignment can be easily arranged. This can be done by locking the receive and transmit polarizations together at the earth station antenna in the same way as at the satellite antenna and then rotating these polarizations by physical or electrical means until ideal polarization alignment is achieved.
If the propagation media affects the polarization attitude of the up and down link waves differently, as it is in the practical case, for instance, due to direction and frequency dependent Faraday rotation through the ionosphere, the two polarizations have to be rotated independently for perfect alignment of the communication system. Such alignment is not possible by simple physical rotation (or equivalent) of the complete antenna and a radio frequency circuit is required which rotates the polarization separately for the up and down link. At the same time these links are at different frequency bands. The present invention provides a system in which the polarization attitudes may be independently and automatically rotated to correct for polarization changes due to Faraday rotation and/or due to rain while maintaining a very high degree of polarization purity and isolation as the rotation of the polarization takes place.